TRANSCRIPTION Copying of the DNA code for a protein into RNA Copying of the DNA code for a protein into RNA 4 Steps: 4 Steps: Initiation Initiation Elongation Elongation Termination Termination Modification Modification

1) INITIATION The enzyme RNA polymerase binds to the DNA molecule at a promoter region, which is upstream from the gene to be transcribed. The enzyme RNA polymerase binds to the DNA molecule at a promoter region, which is upstream from the gene to be transcribed. Upstream – before the gene Upstream – before the gene Promoter region is usually high in A and T Promoter region is usually high in A and T This is because there are only 2 H bonds b/w them, therefore it takes less energy for RNA polymerase to open up the DNA helix here This is because there are only 2 H bonds b/w them, therefore it takes less energy for RNA polymerase to open up the DNA helix here RNA polymerase can only bind at this site, it does not recognize any other sequence. RNA polymerase can only bind at this site, it does not recognize any other sequence.

2) ELONGATION Once bound to the DNA, RNA polymerase breaks the H- bonds, opening the DNA and it also starts building mRNA. Once bound to the DNA, RNA polymerase breaks the H- bonds, opening the DNA and it also starts building mRNA. builds in the direction of 5' to 3'. builds in the direction of 5' to 3'. No primer is needed to start building. No primer is needed to start building. The promoter does not get transcribed. The promoter does not get transcribed.

Template Strand: RNA polymerase uses one strand of DNA as its template for mRNA synthesis Template Strand: RNA polymerase uses one strand of DNA as its template for mRNA synthesis Coding Strand: the strand not used for the template Coding Strand: the strand not used for the template The mRNA will be complimentary to the template strand and identical (except for Us in place of Ts) to the coding strand. The mRNA will be complimentary to the template strand and identical (except for Us in place of Ts) to the coding strand.

3) TERMINATION RNA polymerase reaches a terminator sequence mRNA dissociates from the DNA, At this point the mRNA strand is called a primary transcript (not “true” mRNA yet) RNA polymerase is free to bind with another promoter

4) POSTTRANSCRIPTIONAL MODIFICATION In order to prevent the mRNA from being degraded as it exits the nucleus, a few modifications need to be made to the primary transcript. In order to prevent the mRNA from being degraded as it exits the nucleus, a few modifications need to be made to the primary transcript. The following modifications occur in prokaryotes and eukaryotes: CAPPING: A 5' cap is added to the start (a modified guanine nucleotide). The cap plays a role in protection and also in the initiation of translation. CAPPING: A 5' cap is added to the start (a modified guanine nucleotide). The cap plays a role in protection and also in the initiation of translation. TAILING: a string of ~200 adenosine nucleotides are added to the 3' end by poly-A polymerase to create a poly-A tail. TAILING: a string of ~200 adenosine nucleotides are added to the 3' end by poly-A polymerase to create a poly-A tail.

In eukaryotic DNA further modifications are made. In eukaryotic DNA further modifications are made. In our DNA we have coding regions (EXONS) and non-coding regions (INTRONS). In our DNA we have coding regions (EXONS) and non-coding regions (INTRONS). The introns sit between the exons, so a primary transcript will have parts that don't code for protein. The introns sit between the exons, so a primary transcript will have parts that don't code for protein. Introns need to be removed before translation or the new protein will not fold properly. Introns need to be removed before translation or the new protein will not fold properly. Spliceosomes (RNA and proteins) remove the introns and join the exons together. Spliceosomes (RNA and proteins) remove the introns and join the exons together.

QUALITY CONTROL Once the modifications have been made the primary transcript is known as the mRNA transcript. Once the modifications have been made the primary transcript is known as the mRNA transcript. It can now leave the nucleus. It can now leave the nucleus. Unlike DNA replication, there is no proofreading enzyme, therefore more errors are made in transcription than replication. Unlike DNA replication, there is no proofreading enzyme, therefore more errors are made in transcription than replication. Not a big problem, since hundreds of transcripts are being made (vs. one copy of DNA) and therefore enough protein will be synthesized. Not a big problem, since hundreds of transcripts are being made (vs. one copy of DNA) and therefore enough protein will be synthesized.